In electrical installations, as they comprise, for example, fuel cell applications, electric and hybrid vehicles or also photovoltaic plants, it is generally necessary to transmit electrical power between various components of said installations. Electrical connecting elements are used for this purpose. Various requirements play a role in the selection of suitable connecting elements. It is therefore necessary to also take into account, for example, mechanical requirements and possibly electrodynamic effects, in addition to the electrical power to be transmitted.
In order to transmit high powers, solid busbars, for example of copper or aluminum, are widely used. It is thus customary to also use busbars, for example, for connecting a multi-phase, often embodied as a three-phase, AC voltage output of an inverter to other electrical components of an electrical installation.
A common requirement of electrical connecting elements of this kind is for them to be capable of compensating for inaccuracies in the relative positioning of the components that are to be connected with respect to one another, hence to make a so-called tolerance compensation possible. This is not possible, or is only possible to a limited extent, using the solid busbars mentioned above. For this reason, mechanically flexible connecting elements are preferably used, which may consist of aluminum or copper braid, for example (cf. DE29703525U1, for example). These are often custom-made products that can cause correspondingly higher costs than would arise when using conventional materials.
In all the above-mentioned connecting elements, physical effects, such as what are known as the skin and proximity effects, can lead, when used in AC systems, to the current density within the connecting elements not being homogeneous, in contrast with DC systems. If, for example, in the case of a solid cylindrical conductor, the conductor diameter is substantially larger than the so-called penetration depth that results from the skin effect, in some circumstances, depending on the frequency, only one layer of the thickness of the penetration depth below the surface of the conductor carries the entire current. When using a plurality of conductors, the proximity effect leads to a further displacement effect as a result of eddy currents caused by the voltages induced by changing magnetic fields in adjacent conductors. In the case of conductors that are arranged in parallel and through which a current flows in the same direction, the superposition of the two effects causes the flow of current to be limited to an even smaller cross-sectional area of the conductor. The resulting AC resistance is accordingly high in comparison to the DC resistance. These effects and their consequences for the AC resistance of power cables are illustrated, for example, in R. Suchantke, H. Just, “Numerische Untersuchung des Wechselstromwiderstandes von Energiekabeln” [Numerical investigation of the AC resistance of power cables], TU Berlin, Field of theoretical electrical technology, PROJECT ELECTROMAGNETIC SIMULATION: AC RESISTANCE OF POWER CABLES, Mar. 13 2014.
Said displacement effects thus lead to losses on account of the resulting resistances and thus also lead to local thermal loading within the connecting elements. In this case, the order of magnitude of the loading depends, inter alia, on the frequency of the currents flowing through the connecting elements and on geometric factors, such as the diameter or the spacing of the connecting elements from one another.
In the annual report for 2011 from the Institute of Electrical Power Systems and High Voltage Engineering at the Technical University of Dresden, there is an article by R. Adam entitled “Untersuchungen zur Stromverdrängung in Sammelschienensystemen von Niederspannungs-Schaltgerätekombinationen (NS-SK)” [Investigations into current displacement in busbar systems of low-voltage switching device combinations (LV-SC)]. Said article contains results of a numerical simulation for current density distribution in busbars of a three-phase system that are arranged in parallel with respect to one another, taking into account skin and proximity effects. They show the non-homogeneous current distribution within a design having six busbars through which a three-phase current flows. It is possible to identify here, inter alia, an increased current density in the edge region of opposite busbars of different phases.
U.S. Pat. No. 3,178,668A indicates, inter alia, the possibilities for reducing the impedance in multi-phase energy transmission systems by way of a narrow spatial arrangement of busbars of individual phases.
US20100051342A1 proposes the arrangement of magnetic shields between busbars in order to reduce proximity effects caused by the current flowing through the busbars arranged in parallel.
The prior art contains various approaches to reduce the influence of the above-mentioned displacement effects by way of a suitable internal design of conductors or connecting elements or to reduce the material requirements of these elements, for example, by using hollow conductors (cf. DE10 2012107751A1, for example).
So-called Milliken conductors are therefore used in some applications. Said Milliken conductors are constructed from stranded, electrically isolated segments, which themselves consist of stranded individual wires. What this is intended to achieve as a result is that current of an equal magnitude flows through each individual wire and therefore the voltages induced by the current flows in the particular individual wires can be mutually compensated. The conductors are generally additionally encased by an isolation layer and a copper or aluminum shield (cf. DE102013100955A1, for example).
In addition, wire braids are also used as connecting elements, said wire braids having advantages with respect to busbars but also with respect to the mentioned Milliken conductors as seen from a mechanical point of view by way of their flexibility (cf. DE29703525U1, for example). By using wire braids of this kind, it is possible at the most to reduce the inhomogeneity of the current distribution within the connecting elements caused by the above-described displacement effects. The conventionally used aluminum or copper braids are usually custom-made products that cause correspondingly higher costs than would be the case when using conventional materials.